1. Field of the Invention
This invention relates a function checking system which checks the reel braking function of a magnetic tape cassette.
2. Description of the Prior Art
Generally, a magnetic tape cassette such as a video cassette is provided with a brake mechanism which prevents rotation of the reels when the cassette is not in operation so that the magnetic tape wound around the reels does not unwind from the reels. That is, the outer periphery of the reel is toothed so that top lands and bottom lands are alternately formed, and a brake lever is provided so that it can be swung between an engaging position in which it abuts against one of the bottom lands and engages with a flank of one of the teeth on opposite sides of the bottom land so as to prevent rotation of the reel, and a retracted position in which it is away from the teeth and permits rotation of the reel. The brake lever is urged toward the engaging position under the force of a spring. When the cassette is loaded into a recording and reproducing system, the brake lever is swung to the retracted position overcoming the force of the spring by a suitable means such as a pin provided in the recording and reproducing system.
In the assembling step of the cassette, it is checked whether the brake mechanism will correctly function. Conventionally this check has been accomplished by a visual inspection of an operator, and accordingly, the checking efficiency and the checking accuracy have been unsatisfactory. There have been made various attempts to automate the check. In one attempt, the brake lever is swung overcoming the force of the spring by, for instance, a pusher pin and the load on the pusher pin (counterforce of the brake lever) is detected. Whether the brake mechanism will correctly function is determined on the basis of the comparison of the detected counterforce of the brake lever with a preset value.
More particularly, the pusher pin swings the brake lever under the force of a spring, and the counterforce of the brake lever is determined in terms of the displacement of the pusher pin at the time when the force of the spring urging the pusher pin comes to balance with the force of the spring urging the brake lever. When the displacement is between an upper limit value and a lower limit value, it is determined that the brake mechanism will correctly function. Depending on the type of the cassette, the width between the upper limit value and the lower limit value is very large, and the upper limit value can be six times as large as the lower limit value as shown in FIG. 7. For instance, in one type of cassette, the force of the brake lever urging spring may be between about 10 g and 70 g. Accordingly, if the displacement of the pusher pin is detected by a single detecting means, the error in detection becomes too large and, therefore, each of the upper and lower limit values must be detected by an exclusive detecting means, which add to the cost of the function checking system.
Further, there is a problem of error in detection due to the following interference. Since each of the bottom lands on the outer periphery of the reel has a width in the peripheral direction of the reel, the engaging portion of the brake lever can be moved in the peripheral direction of the reel within the bottom land. Accordingly, depending on the position of the engaging portion within the bottom land at the time the check is effected, the engaging portion can interfere with the corner of the top land adjacent to the bottom land. When the engaging portion is engaged with the corner of the top land, the load which acts on the pusher pin cannot be correctly detected.
Further, since the movable parts in the cassette including the brake lever are generally molded resin articles, the movement of the brake lever can be adversely affected by fin, dust or the like the first time. This can lead to misjudgement that a normal brake mechanism is mistaken for an abnormal brake mechanism.
Though the problems described above may be overcome by use of a stronger spring for urging the pusher pin, this make it more difficult to detect the counterforce of the brake lever which is close to the lower limit value.
Further, use of a load measuring instrument such as a strain gauge instead of the checking system described above leads to a significant increase in cost (e.g., more than ten times).